1. Field of the Invention
This invention relates to a method of an apparatus for distinguishing the types of pixels making up an original, and more particularly to a method of and an apparatus for distinguishing the types of pixels making up an original, which are used, for instance, in a stencil printer which makes a stencil by imagewise perforating a heat-sensitive stencil material and a print is made from the stencil, and a copier or a printer in which a latent image is formed on a photosensitive medium by electrophotography or the like and a toner image obtained by developing the latent image is transferred to a printing paper or a heat-sensitive paper.
2. Description of the Related Art
In the fields of plate making and printing, there has been put into practice a process in which an original, in which binary images such as a line drawing and characters and tone images such as a picture and halftone dots mingle together (such an original will be referred to as a “complex original”, hereinbelow), is read by an image scanner, a multivalued image signal sampled in a main scanning direction and a sub-scanning direction pixel by pixel is obtained, the multivalued image signal is converted into a binary image signal, and a plate or a print is made from the binary image signal.
In order to obtain a desirable output when a complex original is output through an image processing apparatus, generally, the region of the binary image is subjected to a binary image density conversion in which the density of each pixel is converted to a maximum density or a minimum density on the basis of a single threshold value, the region of the tone image is subjected to a tone image density conversion in which the density of each pixel is converted taking into account the properties of the input/output system so that the tone properties of the original image are preserved, and then the region of the binary image is binary-coded by a simple binary-coding method using a single threshold value while the region of the tone image is binary-coded by a pseudo-halftone expressing method such as a dither method and an error diffusion method. Further, since moiré is apt to be generated when a halftone picture is binary-coded by the dither method, it is not preferred that the picture region and the halftone region are subjected to density conversion by the use of the same characteristics and binary-coded by the same method.
Accordingly, it is necessary to determine the type of each region, that is, to distinguish the binary image region from the tone image region such as a halftone region and a picture region. In other words, it is necessary to distinguish the type of each pixel (the type of image which the pixel makes up) and to carry out on each pixel image processing suitable to the type of pixel. For example, pixels of a binary image should be subjected to a density conversion for a binary image so that characters are output to be high in density and pixels of a tone image should be subjected to a density conversion for a picture or a halftone image so that the tone properties of the original image are preserved.
When a complex original is used, it is necessary to precisely distinguish the types of pixels, that is, to distinguish pixels of a character, pixels of a picture or pixels of a halftone image from each other, for an image signal of one frame corresponding to one sheet of original, and to carry out image processing optimal to each type of pixels. There have been proposed various methods of distinguishing the character region, the picture region and the halftone region from each other.
For example, as a method of distinguishing whether a pixel is of a halftone region, there has been proposed a method in which whether a pixel is of a halftone region is determined on the basis of the number of edges in a reference region of a predetermined size. See, for instance, Japanese Unexamined Patent Publication Nos. 2(1990)-274174 and 5(1993)-344331.
In the method disclosed in Japanese Unexamined Patent Publication No. 2(1990)-274174, in an image signal made up of image signal components corresponding to a plurality of pixels arranged in a main scanning direction and a sub-scanning direction, whether or not the pixel is of an edge is determined pixel by pixel, the number of edge pixels (pixels of an edge) in a block including M pixels in the main scanning direction and N pixels in the sub-scanning direction, and it is determined that all the pixels in the block are a pixel of a halftone image or only the pixel at the center of the block is a pixel of a halftone image when the number of edge pixels are larger than a predetermined threshold value th. That is, in this method, whether each pixel is a halftone pixel (a pixel of a halftone image) is determined on the basis of the number of edge pixels in each block.
However, the method disclosed in Japanese Unexamined Patent Publication No. 2(1990)-274174 is disadvantageous in that when characters and/or line drawings coexist with halftone pictures, the character region including therein characters and line drawings can be mistaken for the half tone region. This is because when result of the edge detection in the character region and that in the halftone region are compared with each other, the number of edges in the block or the edge density (an edge characteristic value) are often the same in the character region and the halftone region though edges are successively detected in the character region whereas edges are detected at regular intervals in the halftone region, and the character region and the halftone region are different from each other in arrangement of edges. Accordingly, a part in which a number of small characters such as newspaper characters are arranged can be mistaken for a halftone region and a line drawing region in which many parallel lines are arranged in a row as in a scale can be mistaken for a halftone region.